A conventional selective epitaxy process involves a deposition reaction and an etch reaction. The deposition and etch reactions occur concurrently with relatively different reaction rates to an epitaxial layer and to a polycrystalline layer. During the deposition process, the epitaxial layer is formed on a monocrystalline surface while a polycrystalline layer is deposited on at least a second layer, such as an existing polycrystalline layer and/or an amorphous layer. However, the deposited polycrystalline layer is generally etched at a faster rate than the epitaxial layer. Therefore, by changing the concentration of an etchant gas, the net selective process results in deposition of epitaxy material and limited, or no, deposition of polycrystalline material. For example, a selective epitaxy process may result in the formation of an epilayer of silicon-containing material on a monocrystalline silicon surface while no deposition is left on the spacer.
Selective epitaxy processes generally have some drawbacks. In order to maintain selectivity during such epitaxy processes, chemical concentrations of the precursors, as well as reaction temperatures must be regulated and adjusted throughout the deposition process. If not enough silicon precursor is administered, then the etching reaction may dominate and the overall process is slowed down. Also, harmful over etching of substrate features may occur. If not enough etchant precursor is administered, then the deposition reaction may dominate reducing the selectivity to form monocrystalline and polycrystalline materials across the substrate surface. Also, conventional selective epitaxy processes usually require a high reaction temperature, such as about 800° C., 1,000° C. or higher. Such high temperatures are not desirable during a fabrication process due to thermal budget considerations and possible uncontrolled nitridation reactions to the substrate surface.
As an alternative to a conventional selective epitaxy process, previously incorporated U.S. patent application Ser. No. 11/001,774, filed Dec. 1, 2004 describes an alternating gas supply (AGS) process that includes repeating a cycle of a deposition process and an etching process until the desired thickness of an epitaxial layer is formed. Because an AGS process uses separate deposition and etching steps, deposition precursor concentrations need not be maintained during etching steps and etching precursor concentrations need not be maintained during deposition steps. In some cases, lower reaction temperatures may be employed.
For both selective epitaxy and AGS processes, a need remains for a system for efficiently practicing such processes.